Create waste products that the blood carries away.

Each red blood cell contains hemoglobin, which can transport oxygen. ... The cells need oxygen for metabolism, which also creates carbon dioxide as a waste product. The red blood cells then pick up the carbon dioxide and transport it back to the lung. There we exhale it when we breathe out

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)